1. Technical Field
The present invention is related generally to a method for coating or processing reinforcing agents, such as stiff fibers, that are used to increase the toughness of brittle materials, such as ceramics and inorganic glasses and other materials that exhibit the high temperature properties, such as high strength and low ductility, of ceramics. More particularly, the present invention is related to a new method for coating reinforcing agents, to the coated reinforcing agents produced by said method, and to composite materials that contain said coated reinforcing agents.
2. State of the Art
Ceramics and other brittle materials, such as inorganic glasses, retain strength and low ductility at high temperatures but exhibit low toughness, i.e., resistance to crack propagation. Although they exhibit high strength, it is readily lost through surface damage. Reinforcing agents can be added to such materials in order to increase the toughness of such materials. The introduction of a reinforcing agent into a ceramic or other brittle material produces a composite material, which consists of the reinforcing agent and a matrix of the brittle material. The composite material has the desirable properties, such as strength at high temperatures of the unreinforced material, but has greatly increased toughness compared to the unreinforced material These advantageous properties of reinforced materials, such as toughness and strength at high temperature, can be lost, however, if the reinforcing agent reacts chemically with the matrix or with the environment. Coating the reinforcing agent with a suitable material, such as alumina, minimizes reactions of the agent with the matrix and/or with the environment during processing and use of the material.
Fiber reinforced ceramics consist of a ceramic, glass-ceramic, inorganic-glass or other brittle matrix that is reinforced with a fiber or other agent, such as a whisker or platelet. Typical reinforcing agents consist of high modulus carbon, silicon carbide, silicon carbide deposited on carbon filament, .alpha.-alumina, alumina-borosilicate, boron, tungsten, and niobium stainless steel (see, e.g., Phillips (1983) at pp. 373-428 of Handbook of Composites, vol. 4, Ed. Kelly et al. Elsvier Science Publishers, B.V.).
Numerous processes exist for the fabrication of fiber reinforced ceramic composites. These processes generally involve two stages: incorporation of the fibers into the unconsolidated matrix and consolidation of the matrix. Hot-pressing is the most widely used technique for preparing fiber reinforced ceramic composites. Hot-pressing achieves full density and good mechanical properties in the resulting composite. Complex shapes, however, cannot be readily fabricated by hot-pressing. As an alternative, pressureless sintering is often used for fabricating near-net-shape components. This method is, however, impeded by the stress generation that is caused by differential shrinkage between the ceramic matrix and the reinforcing agents.
In situ observations of the sintering process for composites of silicon carbide fibers (hereinafter SiC.sub.f) in an alumina matrix reveal that these stresses initiate during the heating cycle before the actual sintering temperature of 1450.degree. C. is reached (see, e.g., Ostertag, C.P. (1987) J. Am. Cer. Soc. 70 (12): C355). Because of the low matrix density and consequent low matrix strength at the point of stress initiation, stresses that develop during the early stage of sintering are found to be the most detrimental to achieving a damage-free composite (see, e.g. Ostertag, C.P. (1989) in Sintering of Adv. Ceram., Ceram. Trans., ed. by C. N. Handwerker et al., Am. Ceram. Soc.). Therefore, it is necessary to delay stress development until the matrix density is sufficiently high to withstand the stresses associated with the reinforcing fibers. There is, thus, a need to develop processing routes that delay stress generation during the early stages of sintering, specifically during the heating cycle.
The fiber reinforced ceramic composite that is produced by the addition of a fiber component to a ceramic matrix offers enhanced fracture toughness and strength. The toughening of the fiber reinforced ceramic composite occurs because of an increase in the fracture energy of the reinforced ceramic matrix compared to that of the corresponding monolithic matrix. Further, crack deflection around the fiber increases the stress that is required to break the material and/or increases the amount of energy that is needed to pull the fiber from the surrounding matrix. This in turn increases flaw tolerance in the fiber reinforced ceramic composite, which results in graceful failure thereof.
In a fiber reinforced ceramic composite there exist three layers of materials: the fiber, the interface at the fiber and matrix, and the matrix. The interface at the fiber and the matrix plays an important role in imparting toughness to the composite material because the chemical and physical properties of the interface affect the thermodynamics and kinetics of the reactions of the overall composite system. Coating the reinforcing agent with an appropriate material can be used to minimize reactions between the agent and the matrix and/or the environment during processing and use. According to Kerans et al. (in the Ceramic Bulletin, Vol. 68, No. 2, 1989) the fracture toughness and strength increases of the composite are a direct consequence of fiber reinforcements that have a higher modulus and strength than the matrix. This increase in fracture toughness and strength can only be realized, however, if the interface is able to transfer load from the matrix to the reinforcing fiber.
Thus, optimization of the mechanical properties of the interface has become a key factor in developing reinforced composite materials, such as fiber reinforced ceramic composites. The interface must have properties that maximize the fracture strength and toughness of the composite. Since the choices of fiber and matrix are limited, optimization of the mechanical properties is achieved by the methods used to coat or process the fibers or other reinforcing agents. Several methods have been developed and are currently being used for coating reinforcing agents. These methods include slurry formation followed by fusing, sol-gel, chemical vapor deposition and thermal spraying (see, e.g., Schmid et al. (1988) Ceram. Eng. Sci. Proc. 9 (9-10): 1089-94). None of these methods, however, permit sufficient control of interface chemistry to optimize the mechanical properties thereof.
There is, thus, not only a need to develop processing routes that delay stress generation during the early stages of sintering, but a need to develop coating and processing methods that permit the control of interface chemistry and thereby improve the strength and toughness of the composite material.